For renewable energy such as PV (photovoltaic) and stabilization of an electric power system, and EV (an electric vehicle), etc., the role of a storage battery has been becoming very important. In order to respond to various usage situations, a storage battery system is a form in which a storage battery having a plurality of battery cells (hereinafter referred to as cells) configured to be connected to each other (battery pack) is used. The storage battery system is desired to be able to be used safely and with confidence, and it is important for the storage battery system not to be easily degraded as a whole.
Recently, with electronization of all electronic devices and living environments, large capacity has been increasingly desired for the storage battery system, and the number of connected cells that constitute one storage battery system has become extremely large. Today, a storage battery system including on the order of several thousand cells is used for an EV.
In the storage battery system including a storage battery constituted by a plurality of cells, due to variation in individual composition of each of the cells, the degree of degradation varies for each of the cells by repeated charging and discharging. In a conventional storage battery system, when one cell is degraded, it is necessary that capacities of normal cells be made uniform with the degraded cell, so that the performance of the whole storage battery system is degraded, and the degradation may further cause the failure of the storage battery system itself. In this case, there are also a plurality of cells that are not degraded yet in the storage battery system, and the storage battery system is forced to be replaced undesirably before these normal cells run out of the performance.
On the other hand, in order to save such a waste, it is conceivable that the performance of the storage battery system is recovered by replacing the degraded cell. However, in a large-scale storage battery system, for example, it is difficult to detect which cells are notably degraded because a storage battery including several tens of thousands of cells is mounted therein. If a degraded cell is sequentially searched for, deterioration diagnosis is needed to be performed on several tens of thousands of cells, so that the searching work unfortunately becomes very time consuming.